The field of the invention is that of illumination devices and especially illumination devices coupled with a transmissive display screen of the liquid crystal screen type.
Generally, liquid crystal screens are made by means of technologies of photolithography. They associate a control matrix with a matrix of electrodes controlling elements with variable birefringence. Each element is bordered by a colored filter and polarizing sheets so as to achieve the amplitude modulation of only one color. In natural light, the transparency of a screen of this kind is in the range of 5%. In appropriately polarized light, this transparency is doubled.
This is why, it becomes necessary to use light sources, the most efficient of which are luminescent tubes in which an electrical discharge is an emitter of ultraviolet light, converted into the visible range by a photoluminescent phosphorus deposited on the envelope or casing of the tube. The efficiency of these tubes is typically about 80 lumens/watts. The maximum luminescence of the tube is about 30,000 candela/m.sup.2 in the white.
The means of distributing the light of the tube throughout the surface of the screen are shown in FIG. 1.
The tube 1 is coupled by a reflective envelope 2 to a waveguide 3, consisting for example of a plastic foil with a thickness of a few millimeters. The light is extracted from the waveguide by scattering points 4 distributed on the surface of the film.
The assembly can be complemented by a back-scattering device 5 to increase the light flux extracted towards the screen 6.
In this type of configuration, the balance of luminance and the balance of flux may typically be as follows:
2-watt tube 180 lumens 60 cm.sup.2 Sr 30,000 Cd/m.sup.2 PA0 Input of waveguide 120 lumens 40 cm.sup.2 Sr 30,000 Cd/m.sup.2 PA0 Waveguide surface 120 lumens 1000 cm.sup.2 Sr 1200 Cd/m.sup.2 PA0 Display output 6 lumens 1000 cm.sup.2 Sr 60 Cd/m.sup.2
The major problem encountered in this type of illumination device lies in the low efficiency of extraction of the light, said light being furthermore not polarized at output while the liquid crystal matrices work with polarized light.
Hence, to be able to illuminate a transmissive screen with sufficient energy, in the desired direction, namely perpendicularly to the plane of the waveguide and the plane of the screen, it is indispensable to provide high-powered light sources for supplying the device.
Now, at the present time, it is increasingly being sought to obtain substantial reductions in the amount of electrical power consumed in this type of device. Indeed, in the field of portable screens, the weight of the batteries permitted limits the autonomy of operation. This need to limit power consumption also exists in the field of automobiles or avionics, where the screens are integrated into dashboards and where excessive electrical consumption may result in substantial heating.
In this context, an object of the invention is an illumination device enabling the amount of electrical power consumed to be greatly reduced by means of improved efficiency of extraction of light, this result being obtained by means used to extract polarized light in a useful direction.